COPD = Chronic obstructive pulmonary disease.

Obstructive disease is a distinction between that and restrictive lung
disease and can be differentiated with spirometry (PFT's/ pulmonary fonction
testing). In it when you exhale the airways collapse - or obstruct -
(usually doe to the loss of tethering from the lung tissue (when healthy
"springy" lung tissue hols the airways open) leading to wheezing and or
shortness of breath. A characteristic pattern is seen on the spirogram and,
in severe cases, lung expansion is seen on chest x-rays (both due to the
decrease in healthy lung tissue (less "springiness" and air getting trapped
behiond collapsed airways). In restrictive lung diseases the lung tissue is
stiffer and just can't expand. COPD is by definition chronic meaning that it
does not come and go but it can get better or worse.

Emphysema is a type of COPD resulting from desctruction of lung tissue.
Smoking is usually the cause but there are some other causes such as
occupational exposures (coal miners etc) and enzyme defects (alpha-1
anti-trypsin deficiency). When you look at healthy lung tissue under a
microscope it looks like a honey-comb. Emphysema looks like a honey-comb
where a lot of the walls have been knocked out.

Chronic bronchitis usually affects the larger airways and usually involves
repeated infections from the damage to the lining of the lungs. Sometimes
these airways can actually expand and cuase fluid to sit around where it
normally wouldn't be and then get repeatedly infected (bronchiectasis).

Some authors list asthma in the general class of COPD but most would say
that since it is by definition reversible that it is not (it is not
chronic - it is episodic). The spirometric findings are the same as in COPD
but they normalize either spontaneously or after giving albuterol. Normal
spirometry can rule out COPD but not asthma (unless it is done while
symptoms are present). The fact that asthma shows, at least at times, normal
spirometry can make it hard to diagnose. Of course, long standing severe
asthma can become chronic and not completely reversible.

It would probably make sense if we started calling asthma ROPD (reversible
obstructive pulmonary disease) but I don't see that happening any time soon.